Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: Virtual functions used in constructor illegal ! ??
Keywords: virtual functions, constructor
Message-ID: <59509@microsoft.UUCP>
Date: 3 Dec 90 19:10:45 GMT
References: <822@orthogo.UUCP>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Distribution: comp
Organization: Microsoft Corp., Redmond WA
Lines: 38

In article <822@orthogo.UUCP> basti@orthogo.UUCP (Sebastian Wangnick) writes:
|I just recognized that my cfront 2.0 installs the vtable
|of a derived class only *after* the constructor of the
|base class is called.
|
|This implies that you are not allowed to use virtual member
|functions within the constructor of an abstract base class.
....
|Now: Is this a bug or a feature ??

This is a "feature" -- IE this is how the C++ language is defined.  While
in the base class constructor your object is only a base object, it does
not become a derived object until the derived constructor begins to work
on it.  Compilers may rely on this language definition to play with the
nasty bits such as vtable and vbase pointers during construction.  Thus,
attempting to defeat this language "feature" will be problematic and 
non-portable at best.

Likewise in destructors, while in the derived class destructor your object
is a derived object, but afterwards when the base destructor is called
your object has regressed to becoming just a base object again.

These "features" allow base class constructors and destructors to be
implemented in sensible ways despite what future class implementors might
do in derived classes.  Even the permission to use derived class virtual
functions in the derived class constructor could be viewed as problematic,
since your derived class object does not fully become such until the
end of the derived class constructor.  However, it was assumed that
derived class writers could reasonably be held responsible for their
own actions in using their own virtual functions within their own 
constructors.  But beware: deamons lurk in wait for those who so use
virtual functions!

[In my opinion, one is best not to do substantive work inside a constructor
in the first place.  Constructors are best used to do things to 
place an object into the simplest mode where that object can meet its
class invariants.  Do your substative work, such as virtual function calls,
after the constructors [standard disclaimer]]


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